The helical structure of the human umbilical cord has been studied on 528 full-term cords from normal deliveries, 18 cords from aborted and developmentally normal fetuses with a CR length of 1.2–19.5 cm, and 10 cords from monozygotic twins. In order to resolve the discrepancy in the nomenclature of the twist direction, a method has been suggested which takes into account the direction to which the fetus must have rotated to produce the twist. The incidence of the right helical pattern was 64.58%, of the left helical pattern 15.15%, and of the mixed patterns 17.43%; the twists were indeterminate in 1.89% and absent in 0.95% of the cords. Details of the mixed patterns are listed. The number of the uniform right twists ranged from 1 to 29 (7.5) and that of the left twists from 1 to 19 (6.7). There exists an exponential relation between the number of the twists and the ratio between length and thickness of the cords. The twists begin to appear during the early part of the 8th week, and their final number is possibly attained soon after the 9th week of development. In view of the absence of concordance in monozygotic twins, the helical nature of the cord is possibly controlled by factors which may be partly genetic and partly environmenta